Method for making textured multilayer optical films

Abstract

Methods and apparatuses are provided for the manufacture of coextruded polymeric multilayer optical films. The multilayer optical films have an ordered arrangement of layers of two or more materials having particular layer thicknesses and a prescribed layer thickness gradient throughout the multilayer optical stack. The methods and apparatuses described allow improved control over individual layer thicknesses, layer thickness gradients, indices of refraction, interlayer adhesion, and surface characteristics of the optical films. The methods and apparatuses described are useful for making interference polarizers, mirrors, and colored films that are optically effective over diverse portions of the ultraviolet, visible, and infrared spectra.

Description

CROSS-REFERENCE TO RELATED APPLICATION [0001] This application is a continuation of U.S. patent application Ser. No. 09 / 809,551, filed Mar. 15, 2001, to be issued on Oct. 26, 2004, as U.S. Pat. No. 6,808,658, which is a continuation of U.S. patent application Ser. No. 09 / 229,724, filed Jan. 13, 1999, now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 09 / 006,288 filed on Jan. 13, 1998, now abandoned, and incorporated herein by reference.FIELD OF THE INVENTION [0002] The present invention relates to apparatuses and processes for making polymeric multilayered films, and in particular to coextruded multilayered optical films having alternating polymeric layers with differing indices of refraction wherein at least one of the polymers is able to develop and maintain a large birefringence when stretched. BACKGROUND OF THE INVENTION [0003] The present invention relates to processes and apparatuses for making polymeric multilayered films, and more particularly...

AI Technical Summary

Benefits of technology

[0015] In yet another method of making a multilayer optical film, the method comprises the steps of: (a) providing at least a first and a second stream of resin, wherein the first stream of resin is a copolymer of polyethylene naphthalate (coPEN) and the second stream of resin is polymethyl methacrylate (PMMA), (b) dividing the first and the second streams into a plurality of layers such that the layers of the first stream are interleaved with the layers of the second stream to yield a composite stream; (c) coextruding the composite stream through a die to form a multilayer web wherein each layer is generally parallel to the major surface of adjacent layers, wherein the coPEN and PMMA resins are coextruded at a melt temperature of about 260° C., and wherein the birefringence of the coPEN resin is reduced by about 0.02 units or

Problems solved by technology

These devices are very difficult and costly to manufacture, and are limited in practical terms to making films of no more than about three hundred total layers.

Moreover, these devices are complex to operate and not easily changed over from the manufacture of one film construction to another.

Films made with the materials and processes heretofore described are generally not practical for uses which require uniformity of reflectivity.

These techniques attempted to introduce layer thickness gradients into an already extruded film, and did not permit precise generation or control of the gradients.

Precise control of volumetric flow rates using such a device is difficult to achieve.

One problem associated with microlayer extrusion technology has been flow instabilities which can occur when two or more polymers are simultaneously extruded through a die.

Such instabilities may cause waviness and distortions at the polymer layer interfaces, and in severe cases, the layers may become intermixed and lose their separate identities, termed layer breakup.

Even modest instabilities in processing, resulting in layer breakup in as few as 1% of the layers, may severely detract from the reflectivity or appearance of an article.

To form highly reflective bodies or films, the total number of layer interfaces must be increased, and as the number of layers in such articles is increased in the coextrusion apparatus, individual layer thicknesses become smaller so that the breakup of even a relatively few layers can cause substantial deterioration of the optical properties on the article.

Problems of layer breakup are especially severe for multilayered bodies in which individual layer thicknesses approach about 10 μm or less adjacent to the walls of the feedblock, multiplier, or extrusion die.

Processes described heretofore typically are not able to exploit the potential of the new resins available and do not provide the required degree of versatility and control over absolute layer thickness, layer thickness gradients, indices of refraction, orientation, and interlayer adhesion that is needed for the routine manufacture of many of these films.

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